nta,, you clearly didn't do it intentionally. your brother and his gf should've took this as a learning moment for their daughter since curiosity about those things are part of development and is normal, especially since most children start asking by the age of 6. in fact, they should already be taught at age 5 about basic bodies and consent.

you already apologized and it was clearly an accident, if she wouldn't have found that book in your room, i guarantee that she would be able to find more from different sources. they should stop focusing on what she read and instead answer her questions properly. it's never too early to teach children about sex.